She was born in Omaha, Nebraska but migrated to New York, where she earned degrees at Barnard College and Columbia University. She subsequently studied at Oxford University and the London School of Economics before lecturing in the History Department at Columbia. She joined the Socialist Party of America in 1909 and began working in the labor reform movement after leaving Columbia in 1913. She was instrumental in labor-Left reform organisations such as U.S Immigration Commission, The American Association for Labor Legislation and the Rand School of Social Science. She was employed by Sidney Hillman's International Ladies Garment Workers Union as director of the union's Worker's University. Poyntz continued to work within the Socialist-oriented ILGWU after siding with the fledgling CPUSA. During the 1920's Poyntz was on the staff of the Friends of the Soviet Union and International Labor Defense. Later she quit her outside work in favor of internal party activities. In 1934 she dropped out of open party activities and began performing underground party work for Soviet military intelligence (GRU).

Poyntz disappeared from her New York City home in 1937, and a police investigation turned up no clues to her fate. In early 1938 Carlo Tresca, a leading Italian-American anarchist, publicly accused the Soviets of kidnapping Poyntz in order to prevent her defection. He said that before she disappeared, she had come to him to talk over her disgust at what she had seen in Moscow in 1936 in the early stages of Joseph Stalin's Great Purge.

Poyntz's murder was the key to the ultimate unraveling of the alleged CPUSA secret apparatus. Whittaker Chambers said he heard Poyntz had been killed for attempted desertion, and this contributed to his caution when he defected in 1938. Elizabeth Bentley stated that in the late 1930s, Jacob Golos, and later in 1945 KGB officer, Anatoli Gromov, told her Poyntz had been a traitor and was now dead. Bentley's defection at the end of World War II likewise was in part motivated by fear of the example set in the Juliet Poyntz case.